Thermal Relief Design for Through-Hole Pads
A thermal relief is a pad that is connected to a plane by a small number of narrow spokes rather than by a full connection. It exists so that a through hole joint can be soldered without the plane drawing the heat away, and it costs electrical and thermal performance to buy that.
Why the Relief Exists
A pad connected to a large plane conducts heat away from the joint as fast as the iron or the wave supplies it. The result is a joint that does not reach the melting point at the pad, or one that reaches it only after the laminate around it has been overheated.
The relief reduces that conduction to a level the process can overcome. Our joint criteria notes define the joint that the relief is there to make possible.
Points to Confirm at First Article
A parameter that is set once and never re verified drifts, and the drift is usually discovered by a defect rather than by the record. The first article confirms that the setup matches the intent, and it is the cheapest point at which a wrong setup can still be corrected.
The cost of verification is small compared with the cost of a field failure, and it is paid at a point where the product can still be corrected. Documentation exists so that a person who was not present can reproduce the work and reach the same conclusion.
The acceptance criteria should be written before the work starts, so that the decision is made by the specification rather than by the person inspecting. The environment around the process, including temperature, humidity and cleanliness, sets limits on what the process can hold.
A result that cannot be reproduced is not a result, and reproducibility should be demonstrated rather than assumed. The tooling, the material and the profile form one system, and a change to any of them should be assessed against the other two before it is released.
Handling between operations is part of the process, and the damage it causes is often attributed to the operation that preceded it.
Documented Process Control
Sampling is a compromise between cost and confidence, and the sample size should follow from the failure rate that has to be detected. Where a process is at the edge of its capability, the margin should be bought deliberately rather than discovered during production.
Where two operations share a tolerance, the allocation between them should be explicit rather than left to whichever is measured first. The checks that matter are the ones performed on the product rather than on a sample kept for the purpose, because a coupon that travels with the panel is the only evidence about that panel.
Spoke Count and Width
The relief is usually four spokes at right angles, and the width of each spoke is the parameter that sets the trade. Wider spokes conduct more heat away and carry more current, and narrower ones do the opposite.
The width is normally specified as a figure for the design rather than left to a default. A default that suits a signal pad is too narrow for a joint that carries current. Our thermal design notes describe the electrical side of the same decision.
Consequences for Power and Ground
A thermal relief in a ground plane raises the impedance of the connection, which matters at high frequency and matters little at low frequency. On a power path it also adds resistance and a local hot spot.
Where the connection carries current, the relief is often removed and the joint is soldered by a process that can supply the heat. That is a process decision made with the layout rather than after it. Our profile notes describe how the heat is supplied in each case.
Reliefs and the Assembly Process
A wave soldered board with reliefs on every plane connected pad is easier to solder and slower to cool. A reflowed board with pin in paste has the same problem, and the relief is often the only reason the joint forms.
On a board that is assembled by selective soldering, the nozzle supplies more heat and a smaller relief is sufficient. The relief design therefore follows the assembly process, which is a fact that is often settled after the layout is fixed.
Where Not to Use Them
A relief is not used on a pad that must conduct heat into the plane, on a high current path, or on a connection where the impedance matters. In those cases the joint is made with a process that can heat the plane.
It is also not used where the pad is not connected to a plane on the same layer. A spoke pattern on an isolated pad adds nothing and occupies area that the pad could use.
Specifying the Relief
The drawing should state the spoke width, the spoke count and the clearance, and it should say which pads have one. A general note that applies reliefs everywhere produces boards that are easy to solder and electrically different from the design.
Where the requirement differs between pads, the difference belongs in the note rather than in the fabricator’s judgement. Our board quality notes describe how the finished pad is judged.
Checks Before Release
Where a requirement can be measured, it should be measured at the point of manufacture and recorded against the board or the lot it applies to. Where the process window is narrow, the measurement resolution has to be better than the window, or the data cannot distinguish a good part from a marginal one.
A record that identifies the operator, the date and the settings is worth more than a record that identifies only the result.
FAQ
Does a relief reduce the current capacity? It does, and the reduction is the ratio of the spoke area to the full connection area.
Is a relief needed on a two layer board? It is if the pad connects to a plane on the same layer, and it is not if the plane is on the other side.
What does gopcb provide for thermal reliefs? We provide spoke width and count stated per pad type, clearance defined with the mask, reliefs applied only where the drawing asks for them, the assembly process considered in the design, and the finished pad verified against the drawing.
